Significant recent interest in full-field velocity imaging techniques has focused on speckle velocimetry or particle image velocimetry techniques. See, e.g., Roland Meynart "Instantaneous Velocity Field Measurements in Unsteady Gas Flow by Speckle Velocimetry," Applied Optics, Vol. 22, No. 4, 15 Feb. 1983, pp. 535-540; Ronald J. Adrian, et al., "Development of Pulse Laser Velocimetry (PLV) for Measurement of Turbulent Flow," Symposium on Turbulence, Univ. of Missouri, Rolla, 1984, pp. 170-186; C. C. Landreth, et al., "Double Pulse Particle Image Velocimeter With Directional Resolution for Complex Flows," Experiments in Fluids, Vol. 6, 1988, pp. 119-128. Despite the different terminology of some authors, most of these techniques are similar. The techniques evaluate particle velocities from double exposure photographs separated by a known time differential. For conditions in which the particle density is large, a common analysis technique for the double exposure particle photographs is to pass an unexpanded laser beam through the double exposure image. The particles which fall within the beam scatter the beam, with particle image pairs producing the well known pattern of Young's fringes. Depending on the conditions under which the particle images were produced, the sets of particle pairs within the probe beam may exhibit a range of apparent displacements. Average displacement and direction information may be obtained by analyzing the Young's fringe pattern for the fringe spacing and fringe angle. Information concerning the details of the variance of the displacement of the velocity pairs within the probe beam from the mean displacement is carried by the modulated shape of the Young's fringes, but is difficult to extract directly. See, K. Hinsch, et al., "Fringe Visibility in Speckle Velocimetry and the Analysis of Random Flow Components," Applied Optics, Vol. 23, No. 24, Dec. 15, 1984, pp. 4460-4462.
An alternative processing technique has been proposed which acquires the two dimensional spatial Fourier transform of the Young's fringe pattern, providing correlation peaks in the frequency plane at the frequency and angle corresponding to the fringe pattern of Young's fringes. See, Ronald J. Adrian, et al., supra. The shape of the correlation peak, as well as the overall background level, can be calculated to provide quantitative estimates of the velocity variance at the particular laser beam probe point.
A typical arrangement for carrying out the forgoing technique includes a laser beam positioner, the image of interest, and some type of high resolution image capture device, such as a 512 x 512 charge coupled device (CCD) array. For each beam position corresponding to a position in the flow field, a fringe pattern is produced, recorded, and numerically transformed. However, numerical transformation of 512 x 512 arrays can be time consuming, even for relatively specialized processors. A typical field may require 10,000 (100 by 100) interrogation points, and an unsteady flow may require many images, depending on the levels of detail required. Consequently, a time consuming series of digital transforms are required.